Electrode second type

Rutherford found that a second type of radiation was attracted to the positively charged electrode. He proposed that this type of radiation consists of a stream of negatively charged particles. By measuring the charge and mass of these particles, he showed that they are electrons. The rapidly moving electrons emitted by nuclei are called (3 particles and denoted (3". Because a (3 particle has no protons or neutrons, its mass number is 0 and it can be written Je. 

A silver electrode in aqueous KCl solution is the example of an electrode of the second type. Here the reaction... 

Electrodes of the second type can formally be regarded as a special case of electrodes of the first type where the standard state (when E = °) corresponds not to flAg+ = 1 but to a value of == 10 mol/L, which is established in a KCl solution of unit activity. In this case, the concentration of the potential-determining cation can be varied by varying the concentration of an anion, which might be called the controlling ion. The oxides and hydroxides of most metals (other than the alkali metals) are poorly soluble in alkaline solutions hence, almost all metal electrodes in alkaline solutions are electrodes of the second type. 

The classification of electrodes is based upon the chemical nature of the substances participating in the electrochemical process [75]. Electrodes of the first type are systems in which the reduced forms are metals of electrodes and oxidized forms are ions of the same metal. Electrodes of second type are systems in which the metal is covered by a layer of low soluble salts (or oxide), and the solution contains anions of these salts (for oxide-OH ions). The Nernst equation for electrodes of the second type can be written as ... 

Metal oxide electrodes are also of the second type. A well known example is a rod of antimony coated with Sb203 (or bismuth with Bi203), which can function as a pH electrode33 ... 

The second type of polarization, concentration polarization, results from the depletion of ions at the electrode surface as the reaction proceeds. A concentration gradient builds up between the electrode surface and the bulk solution, and the reaction rate is controlled by the rate of diffusion of ions from the bulk to the electrode surface. Hence, the limiting current under concentration polarization, ii, is proportional to the diffusion coefficient for the reacting ion, D (see Section 4.0 and 4.3 for more information on the diffusion coefficient) ... 

A second type of current arises due to the presence of the electrochemical double layer (Sect. 1.2). Additionally, a current may flow due to the adsorption or desorption (Sect. 1.2) or species O and R as well as electroinactive species. In these instances, no chemical reaction occurs and consequently electrons are not transferred across the electrode-solution interface. However, a current may flow elsewhere and this current is called a non-faradaic current. 

Mediators such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) [166], hexacyanoferrate(III) [168] and CoPC [169], which have been applied to a carbon paste or ink, are reduced by the reaction with thiocholine and then reoxidised at the carbon electrode (Fig. 23.6). A second type of mediator has involved the addition of Prussian blue (ferric hex-anocyanoferrate) into an AChE and choline oxidase (ChO) bienzyme biosensor. Prussian blue mediates the reduction of hydrogen peroxide produced by the conversion of choline to betaine by ChO [170]. 

The second type of sample is composed of single crystals [19]. The crystals are plate-like (flakes) and have sub-millimeter size. They were glued by silver epoxy to the sample holder by one of their side faces. The opposite face of the flakes was used as a needle to gently touch the noble metal counter electrode in liquid helium. In this way we tried to make, preferentially, a contact along the ab plane. On average in... 

To model a porous electrocatalyst we may consider a second type of mass transport (in addition to diffusion) locally within the electrode, i.e., a mass transport resistance between the electrode surface and the solution. This situation may arise, for example, when the electrode surface is covered by a thin layer of polymer electrolyte or as in a fuel cell electrode in which the electrocatalyst is also covered by a thin water layer. 

The second type is the W connection. As can be seen in Fig. 7.4b, it is important to be able to illuminate the module from both sides, since photoelectrode and counter electrode layers are alternately deposited on each glass plate. An advantage over the Z design is that it does not require conductive elements between adjacent cells. A disadavantage, however, is that light illumination through the platinum counter electrodes, which must be transparent, normally leads to lower currents. This current deficit can be compensated to some extent by making the counter electrodes wider. 

This type of electrode is a source or sink of electrons, permitting electron transfer without itself entering into the reaction, as is the case for the first or second type of electrodes. For this reason they are called redox or inert electrodes. In reality the concept of an inert electrode is idealistic, given that the surface of an electrode has to exert an influence on the electrode reaction (perhaps small) and can form bonds with species in solution (formation of oxides, adsorption, etc.). Such processes give rise to non-faradaic currents (faradaic currents are due to interfacial electron transfer). This topic will be developed further in subsequent chapters. 

This classification is useful mainly for electrodes of the first and second types. The great majority of electrodes are, however, of the third or fourth types. 

Table 2.1 lists half-reactions for electrodes of the second type and their potential for unit activities. These electrodes have, in the majority of cases, their own electrolyte associated with them. So, to calculate the potential of a cell in relation to the standard hydrogen electrode it is necessary to take the liquid junction potential between the two electrolytes into account (Section 2.10). 

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